Ninja Multi-Config: Shuffle variables again
This change does three things:
- Remove the
NMC_
portion of the variable names - Make it an error to use the variables in other generators (to leave room for future expansion)
- Use the first item from
CMAKE_CONFIGURATION_TYPES
ifCMAKE_DEFAULT_BUILD_TYPE
is not specified
This change was prompted by the following Discourse thread: https://discourse.cmake.org/t/ninja-multi-config-is-overly-complex/684
Backport: release